Overview drug elimination Elimination of drugs rate of elimination is rate of disappearance of active molecules from bloodstream or body rate of elimination and dosage determine duration of action of a drug drug elimination ≠ drug excretion e.g., a drug may be eliminated by metabolism before excretion from body Zero-order elimination Zero-order elimination implies rate of elimination is constant, regardless of drug concentration constant amount of drug is eliminated per unit time drug concentration in plasma (Cp) decreases linearly with time e.g., phenytoin, ethanol, aspirin aspirin is zero-order drug at high therapeutic or toxic concentrations First-order elimination First-order elimination implies rate of elimination is proportional to drug concentration constant fraction of drug is eliminated per unit time drug concentration in plasma (Cp) decreases exponentially with time first-order drugs have a characteristic, constant half-life of elimination